Phonon broadening in high entropy alloys

Abstract

Refractory high entropy alloys feature outstanding properties making them a promising materials class for next-generation hightemperature applications. At high temperatures, materials properties are strongly affected by lattice vibrations (phonons). Phonons critically influence thermal stability, thermodynamic and elastic properties, as well as thermal conductivity. In contrast to perfect crystals and ordered alloys, the inherently present mass and force constant fluctuations in multi-component random alloys (high entropy alloys) can induce significant phonon scattering and broadening. Despite their importance, phonon scattering and broadening have so far only scarcely been investigated for high entropy alloys. We tackle this challenge from a theoretical perspective and employ ab initio calculations to systematically study the impact of force constant and mass fluctuations on the phonon spectral functions of 12 body-centered cubic random alloys, from binaries up to 5-component high entropy alloys, addressing the key question of how chemical complexity impacts phonons. We find that it is crucial to include both mass and force constant fluctuations. If one or the other is neglected, qualitatively wrong results can be obtained such as artificial phonon band gaps. We analyze how the results obtained for the phonons translate into thermodynamically integrated quantities, specifically the vibrational entropy. Changes in the vibrational entropy with increasing the number of elements can be as large as changes in the configurational entropy and are thus important for phase stability considerations. The set of studied alloys includes MoTa, MoTaNb,

(Hf0.2Zr0.2Ta0.2Nb0.2Ti0.2)C high‐entropy ceramics with low thermal conductivity

“…However, an EXAFS analysis will be needed to examine this hypothesis. In contrast to perfect crystals and ordered materials, the mass and interatomic force constant fluctuations in HECs, due to anion sublattice distortion, can induce severe phonon scattering . The low thermal conductivity of (Hf 0.2 Zr 0.2 Ta 0.2 Nb 0.2 Ti 0.2 )C may be mainly attributed to its distorted anion sublattice, at which the phonons are scattered more significantly.…”

Single-Phase Concentrated Solid-Solution Alloys: Bridging Intrinsic Transport Properties and Irradiation Resistance

“…While directly determining the lattice thermal conductivity is non-trivial, it can be achieved from the phonon spectra since it is inversely proportional to phonon bandwidth, which is affected by the mass and force constant fluctuation. A recent theoretical work employing ab initio calculations has systematically studied the impact of these two kinds of disorders in a series of BCC equiatomic alloys, from binary to quinary (refractory HEAs) (Körmann et al, 2017). In this series of calculations, the mass fluctuations have played a dominant role, although the impact from force constant fluctuation could not be neglected.…”

Copper Sulfides: Earth‐Abundant and Low‐Cost Thermoelectric Materials

“…Several approaches are introduced as modern concepts to improve the thermoelectric performance; these are design of alloys, [36][37][38][39][40][41][42][43][44] fabrication of heavy element compound, [12,[45][46][47] complex crystal structuring, [48,49] resonant level doping, [13,50] and so on, aiming to create PGEC. Despite all these classical approaches, there is one more important strategy called nanostructurization, which is proven to be very promising.…”

“…A low thermal conductivity of 0.47 W m −1 K −1 is observed in the BiSbTe 1.5 Se 1.5 alloy due to severe lattice distortions . The large compositional space in high‐entropy alloys offers the possibility of tuning thermal conductivity because of lack of ordering in these materials commonly attributed to the large configurational entropy . So, by adjusting the chemical complexity, such as variation of fractions of the elements, one can systematically modify the thermal conductivity .…”

“…The large compositional space in high‐entropy alloys offers the possibility of tuning thermal conductivity because of lack of ordering in these materials commonly attributed to the large configurational entropy . So, by adjusting the chemical complexity, such as variation of fractions of the elements, one can systematically modify the thermal conductivity . Also, interesting properties are observed through manipulating the structures and electronic band structures in some of the solid alloy solution systems, such as PbTe‐PbSe, PbTe‐PbS, PbSe‐PbS, PbSe‐SnSe, PbTe‐PbSe‐PbS, SnSe‐SnS, and so on, which are considered as important materials for future applications.…”